Water from natural sources often contains dissolved minerals, with an appreciable presence of ions such as Ca2+, Mg2+ and, in the case of oilfield formation water, Ba2+, Sr2+ and Ra2+. Under conditions of temperature or pH change, loss of carbon dioxide from solution or admixture with other water containing different mineral content, relatively insoluble species such as carbonates and sulphates may deposit from solution as scale. In offshore oilfields such deposition may be particularly acute when sulphate-containing seawater, pumped underground to aid oil recovery, comes into contact with formation water.
Deposited scale impedes oil recovery and may even become severe enough to block an oilwell. It is therefore a common procedure to treat oilwells with a scale inhibitor to minimize or prevent scale deposition.
In use, a relatively concentrated solution of the scale inhibitor is pumped down the oilwell and flushed out into the formation. From here it leaches back into the produced water, protecting the well and the pipework from scaling.
A careful balance of properties must be achieved. The scale inhibitor does not only have to control scale, but must also on the one hand have sufficient solubility in the waters at the temperatures it will meet to enable placement in the formation without itself prematurely precipitating from solution, whilst on the other hand it must retain strongly enough with in the formation rock to give a suitable slow leach rate. If the scale inhibitor does not adsorb strongly enough it will all leach back very quickly and the well will require re-treatment after a short time. The retention of scale inhibitor can be achieved by adsorption and precipitation processes. Precipitation is a known method for achieving longer scale inhibitor squeeze lifetimes.